COMPRESSION AND ASSOCIATED PROPERTIES OF BORON CARBIDE

ABSTRACT Our present work presents a direct association of the observed loss of shear strength in boron carbide under plane shock wave compression to amorphization in boron carbide under triaxial stress compression. This evidence is obtained from in-situ measurement of Raman, and infrared vibrational spectra of boron carbide confined in a Diamond Anvil Cell (DAC) under hydrostatic and non-hydrostatic pressures to 50 and 23 GPa, respectively. X-ray-diffraction measurements do show a shift in the compression of boron carbide around 27 GPa. However, X-ray diffraction measurements indicate that the amorphization does not extend to m scale, as there is no evidence of a loss of crystallinity in the recorded diffraction pattern of boron carbide to 47 GPa. Our work shows that shear plays a very dominant role in the stress-induced amorphization of boron carbide

Stability of LLM-172 under high pressure

LLM-172 or 3, 4-bis (4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole has been studied experimentally and computationally modeled at high-pressure. Minimum enthalpy structures were relaxed using norm-conserving pseudo potentials which provided a high level of convergence for the final computational structures. The calculated P-V curve fits reasonably well to the experimental X-ray diffraction data. No phase transitions or deviations from the P212121 (D2-4) space group of the LLM-172 crystal were observed to near 35 GPa, although slight modifications to the molecular geometry were noted in the Raman spectra. Density functional perturbation theory was used to obtain calculated Raman spectra; these calculated spectra were then used for comparison with experimental Raman spectra and the identification of the atomic motions associated with the vibrational modes. Based upon the modification of the experimental Raman spectra with pressure potential decomposition mechanisms are proposed. Quantum mechanical molecular dynamics (QM MD) calculations of LLM-172 surfaces resulted in determination of the very first fragments decomposed from the surface at high temperatures

Cyanoacetohydrazide under Pressure: Chemical Changes in a Hydrogen-Bonded Material

Cyanoacetohydrazide (CAH, C3H5N3O) has been studied under pressure using diamond anvil cell techniques. CAH was characterized using Raman spectroscopy to 30 GPa and synchrotron X-ray diffraction to 45 GPa. The Raman spectra of CAH show reasonable qualitative agreement with first-principle calculations. The X-ray data reveal that CAH maintains its monoclinic structure to approximately 22 GPa with a density change of 12% over this range. Near 22 GPa, the Raman modes and most of the X-ray diffraction peaks disappear. These pressure-induced changes are irreversible upon the release of pressure, and the transformed sample can be recovered to ambient pressure. The recovered sample is photosensitive and shows reaction even at low laser powers of 10 mW at 532 nm. The paper concludes with observations of the roles of hydrogen bonding, molecular configurations, and the behavior of the cyano group in the pressure-induced changes in CAH